The invention is in general directed to an electronic transfer case shifting apparatus which allows shifting between low and high range drive modes provided by the transfer case on-the-fly while the vehicle is moving and under power.
Four-wheel drive systems for vehicles are becoming increasingly popular in a variety of types of vehicles. Four-wheel drive vehicles have typically included a transfer case connected to the output of a vehicle transmission, with manual control means operable in the vehicle passenger cab, enabling shifting between two- and four-wheel drive modes as well as low and high range drive modes. More recently, certain full-time four-wheel drive systems have been employed, wherein a transfer case is provided with an interaxle differential for dividing torque between the vehicle front and rear differentials. Control systems for operating such a transfer case have been developed, and may be used to prevent excessive slipping between front and rear wheels by selectively locking the interaxle differential upon sensing such slippage.
In part-time four-wheel drive vehicles, it is known to use a gear synchronizer clutching arrangement to provide "shift-on-the-fly" shifting between two- and four-wheel drive modes. Such a synchronizer will shift the transfer case into two-wheel or four-wheel drive modes upon synchronization of the speeds of front and rear axles of the vehicle. In four-wheel drive vehicles, the torque transfer case may also provide high and low range drive modes during four-wheel drive operation. Generally, shifting between high and low ranges requires that the vehicle be stopped and that the vehicle transmission be placed into a neutral position, while the brake is applied to insure that the respective vehicle wheels and axles are not rotating. Attempts have been made to install mechanical synchronizers to allow shifting between high and low ranges, but the additional hardware increases costs and complexity of the transfer case.